To record and store a vast quantity of information as typified by moving image information, advanced optical information media such as read-only optical disks and optical recording disks are required to increase their recording density for increasing the capacity. To meet such a demand, engineers have been engaged in the research and development works targeting a higher recording density.
One such approach relating to digital versatile disks (DVD) is to shorten the wavelength of a recording/reading laser beam and increase the numerical aperture (NA) of a recording/reading optical system objective lens, thereby reducing the spot diameter of the recording/reading laser beam. As compared with CD, DVD is successful in achieving a recording capacity of 6 to 8 folds (typically 4.7 GB/side) by changing the recording/reading wavelength from 780 nm to 650 nm and the NA from 0.45 to 0.6.
Increasing the NA, however, leads to a reduced tilt margin. The tilt margin is a permissible tilt of an optical recording medium relative to an optical system, which depends on the NA. The tilt margin is in proportion toλ/(d·NA3)wherein λ denotes the wavelength of recording/reading beam and “d” denotes the thickness of a transparent substrate the recording/reading beam enters. If the optical recording medium is inclined or tilted relative to the laser beam, a wavefront aberration (or coma) occurs. The coefficient of wavefront aberration is represented by(½)·d·{n2·sin θ·cos θ}·NA3/(n2−sin2θ) −5/2wherein n denotes the refractive index of the substrate and θ is a tilt angle. It is appreciated from these formulae that the tilt margin may be increased and the occurrence of comatic aberration be suppressed by reducing the thickness “d” of the substrate. In fact, the DVD design is such that a tilt margin is secured by reducing the thickness of the substrate to about one half (about 0.6 mm) of the thickness (about 1.2 mm) of the CD substrate.
To record moving images of better quality for a longer period of time, there has been proposed a structure allowing for use of a thinner substrate. In this structure, a substrate of an ordinary thickness is used as a supporting substrate for maintaining rigidity, pits or a recording layer is formed on the surface of the supporting substrate, and a light-transmitting layer of about 0.1 mm thick is formed thereon as a thin substrate. Recording/reading beam reaches the pits or the recording layer through the light-transmitting layer. This structure can achieve a higher recording density due to a greater NA because the substrate can be made extremely thin as compared with the prior art. Media having such structure are disclosed in Japanese Patent Laid-Open No. (JP-A) 10-320859 and JP-A 11-120613.
The provision of a light-transmitting layer of approximately 0.1 mm thick allows for use of an objective lens having a large numerical aperzture NA, say about 0.85.
A typical technique used in forming the light-transmitting layer having a thickness of about 0.1 mm is spin coating technique. When the spin coating technique is employed, the disk substrate is secured on the rotating table and the resin is supplied onto the surface of the disk substrate, and the disk substrate is rotated to thereby spread the resin by centrifugal force and form the resin layer. In such process, it is commonplace to use an actinic radiation-curable resin such as UV curable resin which does not require heating for the curing and which cures at a high speed.
However, when the resin is spread by centrifugal force, the resin is likely to be accumulated in the outer peripheral region of the disk substrate, and a bump is likely to be formed in the resin layer. Furthermore, the resin layer is generally cured after terminating the rotation by irradiating an actinic radiation such as UV, and accordingly, the resin after the termination of the rotation is likely to move back to become raised near the outer peripheral edge of the disk substrate by the surface tension. As a consequence, a relatively wide bump is formed near the outer peripheral edge of the disk substrate. The area where such bump has been formed can not be used as a recording area, and this is a serious disadvantage for the recording capacity of the medium particularly in view of the large area per unit length in radial direction of the outer peripheral region of the disk substrate. Accordingly, there is a need to reduce the width of the bump or to avoid the bump formation around the outer peripheral region of the disk substrate.
JP-A 11-73691 discloses a process wherein a photo-curing resin is coated on the substrate to form a resin layer, the bump formed in the outermost peripheral region of the substrate is covered with a mask, and the resin layer is cured except for the area covered with the mask by irradiating the resin layer with curing beam. The uncured region is then flattened by selectively removing the raised part of the uncured resin, and the resin layer is again irradiated with the curing beam to thereby form a cured resin layer which is flat over the entire surface. This process, however, is so complicated requiring two curing steps, attachment/detachment of the mask, and the resin removal, and this complexity renders the process far from being practicable in an industrial scale.
JP-A 11-86355 proposes removal of the bump formed at the outer peripheral edge of the light-transmitting layer by trimming with a diamond wheel. JP-A 11-86356 proposes removal of the bump formed at the outer peripheral edge of the light-transmitting layer by cutting or press shearing. These methods, however, are not only cumbersome but also suffer from generation of burrs and attachment of dusts, which may adversely affect the properties of the optical disk.
JP-A 11-203724 proposes formation of the light-transmitting layer by applying a UV curable resin on a rotating substrate, and irradiating the coating with UV after reducing the rotational speed of the substrate to thereby reduce the size of the bump in the outer peripheral region. JP-A 11-203724 also proposes a method wherein a UV curable resin is dropped onto a stationary substrate, a glass plate is placed on the resin, the substrate is rotated to spread the resin, and the UV is irradiated while rotating the substrate to thereby reduce the size of the bump of the light-transmitting layer. JP-A 11-203724 also proposes a method wherein the substrate is preliminarily cut so that the cross-section of the outer peripheral region of the substrate is in the shape of wedge to thereby reduce the size of the bump in the outer peripheral region of the light-transmitting layer. JP-A 11-203724 also proposes a method wherein, before the coating of the UV curable resin, the outer peripheral region of the substrate is selectively irradiated with a short-wavelength UV to improve wettability of the irradiated area, and then coating the resin to thereby reduce the size of the bump in the outer peripheral region of the light-transmitting layer. JP-A 11-203724 also proposes a method wherein a UV curable resin is spread by rotating the substrate, excessive resin accumulated at the outermost peripheral region is spun off by rotating the substrate at a higher speed, and then, the resin is cured by UV irradiation to thereby reduce the size of the bump in the outer peripheral region of the light-transmitting layer. JP-A 11-203724 also proposes a method wherein a UV curable resin is spread by rotating the substrate, the inner peripheral region of the substrate is selectively irradiated with UV to cure the resin in that region, excessive resin accumulated at the outermost peripheral region is spun off by rotating the substrate at a higher speed, and then, the resin is cured by UV irradiation to thereby reduce the size of the bump in the outer peripheral region of the light-transmitting layer. JP-A 11-203724 also proposes a method wherein the light-transmitting layer is formed after fitting the substrate in a ring-shaped member to increase the diameter of the area to be coated, and removing the ring-shaped member together with the bump of the light-transmitting layer formed thereon to thereby obtain a smooth light-transmitting layer. JP-A 11-203724 also proposes a method wherein the light-transmitting layer is formed on a substrate having a diameter which is larger than the one normally used so that the bump is formed in the outer peripheral region, and cutting off the outer peripheral region of the substrate with the bump formed thereon to thereby obtain a smooth light-transmitting layer. JP-A 11-203724 also proposes a method wherein a UV curable resin is spread, excessive resin in the outer peripheral region of the substrate is removed by rubbing off with a cloth, by sucking with a vacuum pump, or by blowing off with nitrogen, and then, the resin is cured by UV irradiation to thereby reduce the size of the bump in the outer peripheral region of the light-transmitting layer.
Most of the methods disclosed in JP-A 11-203724, however, may suffer from a drop-off in productivity due to increase in the number of steps, increase in the complexity of controlling the production system, and the like.